Pages

Wednesday, October 27, 2010

This month's Accretionary Wedge is being hosted by Matt Kuchta over at Research at a Snail's Pace, and it's all about deskcrops, "the spookier the better." I missed the fisrt Accretionary Wedge on Deskcrops, which was Wedge #4 at Goodschist, although I've featured a few random deskcrops now and again (exactly two, so far). And, as I explained elsewhere recently, I don't really have a desk, and will have to use either shelfcrops from inside the house, garden crops, porchcrops from the steps near the back door, or back-of-truck-crops. I had one vote for garden crops and one vote for porchcrops, but am going with a spooky little indoor shelfcrop.

Scale: Entire rock is 3cm in diameter.

MOH brough this little gem home from a hike one day, and it was immediately in the running for top shelfcrop, seeing as how Halloween is nearly upon us and it resembles an cheerfully smiling jack-o-lantern. It has a big grin, one partly squared off round eye, and one triangular eye. In reality, it's a dark gray, brecciated dolomite (now running out to the truck to get the HCl in order to test this diagnosis - yes!). Some breccia clasts appear to have been rotated or at least moved (the triangular eye) while others appear to be nearly in place (part, or even most, of the smile); the clasts were then cemented by a matrix of white, coarsely crystalline calcite dolomite (HCl to the rescue).

This is a sedimentary rock (dolomite) that has been brecciated under unknown circumstances, given that the context (an outcrop) is missing. Consequently, I don't know what kind of breccia this is. Possibly it's a karst breccia; maybe it's a hydrothermal breccia; maybe it's a sedimentary breccia that was cemented with dolomite. Maybe it's some kind of fault or tectonic breccia. Just don't know. Oh, and then I had to do this, thinking I saw a couple veinlet correlations......and then this......and then this. Purple outlines the breccia fragments; dark pink follows veinlets. And here's our grinning jack-0-lantern breccia in it's current home, on a shelf, leaning against a folded towel, making it truly a shelfcrop, and a top shelfcrop, at that!

Happy Halloween!Or, Happy Nevada Day, depending on which you prefer to celebrate. :)